WO2002/030154A1 discloses a hearing aid comprising a radio-frequency (RF) receiver in which an analog frequency-modulated RF signal in the range from 70 MHz to 250 MHz is picked up by an antenna and amplified in a low-noise amplifier. A mixer mixes the signal down to an intermediate frequency (IF) of 35 kHz. A steep IF filter suppresses undesired signals in adjacent channels. A limiter boosts the IF signal and transforms the analog signal to digital signal levels using a hard-clipping comparator. A digital demodulator detects the zero-crossings of the IF signal, and a decimator transforms the single-bit demodulated signal into a 12-bit signal at a sampling frequency of 24 kHz. The 12-bit signal forms a digital audio output of the receiver.
EP1316240B1 discloses a binaural hearing system with two hearing prostheses capable of performing bi-directional data communication over a wireless communication channel. Wireless transceivers transmit and receive modulated data signals by utilising near-field magnetic coupling between inductive coils. The coils may be tuned to provide a Q for each of the inductive antennas of about 4, preferably between 3 and 10 to optimise the received/transmitted power at the antennas. The communication frequency is preferably selected to a frequency between 50 MHz-100 MHz. An RF demodulator down-converts the received composite RF signal to a baseband frequency range and retrieves the modulated data signal.
These are just two examples of many known RF receivers, which mix a modulated RF signal to a lower frequency, i.e. perform a downward frequency translation, prior to digitising or sampling. This frequency translation makes channel filtering and digitising less critical and allows a substantial reduction of the power consumption. However, the circuits used for mixing the modulated RF signal and for filtering the signal at the lower frequency are mainly analog and thus often deviate from desired specifications due to relatively large component tolerances and temperature drift of the component values. This may lead to a deterioration of the received signal and may thus decrease the obtainable communication speed where the received signal comprises digital information. Furthermore, the analog mixing and filtering circuits are often relatively complex and typically consume a substantial amount of power. It is further not trivial to design analog circuits in which the mixing frequency and/or the filter bandwidth may be changed electronically, which adds to complexity and/or restricts the use of prior art receivers.
It is therefore desirable to perform conversion into a digital signal directly on the modulated RF signal and use digital filters for removing unwanted signal frequencies, followed by digital demodulation. Such “direct sampling” would allow more stable processing of the RF signal, allow a simpler receiver design and further allow changing the characteristics of the wireless connection by reprogramming, so that one and the same circuit could be used in a wider range of applications.
Direct sampling of modulated RF signals has, however, been almost impossible to realise in practice. Normally, the sampling rate in the analog-to-digital conversion must be at least twice the RF signal frequency in order to avoid frequency aliasing, and the digitiser—or analog-to-digital converter (ADC)—needs a high resolution in order to allow retrieval of weak signals in the presence of strong unwanted signals. With the technology available today, these two requirements typically combine to cause unacceptably large power consumption in the digital conversion and filtering circuits. In a typical setup, reducing the power consumption to an acceptable level would therefore require compromising the signal quality to an unacceptable degree.
It is an object of the present invention to provide a receiver for retrieving an information signal from a magnetic induction signal comprising a carrier signal modulated in dependence on the information signal, which receiver is capable of direct sampling of the magnetic induction signal without the above mentioned disadvantages.
It is a further object to provide a use of such a receiver.
It is a further object to provide a method for retrieving an information signal from a magnetic induction signal comprising a carrier signal modulated in dependence on the information signal, which method comprises direct sampling of the magnetic induction signal without the above mentioned disadvantages.